Skin is the largest organ of the body and the interface between the environment and our internal biology. It is composed of two primary layers: the epidermis, which is the outermost layer of skin; and the dermis, which lies just beneath the epidermis. Keratinocytes are the major cells, constituting 95% of the epidermis. The suprabasal keratinocytes differentiate into a chemically and physically resistant horny layer surrounded by proteins and lipids, including ceramides, cholesterol and fatty acids. Natural or forced removal of the top layers of this cornified epithelium will stimulate turnover by the underlying cells to replace the damaged or lost cells. This cornified layer provides the protective and water-barrier functions between the body and the environment. The primary function of keratinocytes is the formation of a barrier to protect the body against chemical, physical and mechanical hazards, invasion by micro-organisms, heat, UV radiation and water loss (Proksch et al., 2008). Keratinocytes are also a main constituent of mucosal tissues that are continuous with the epidermis (Presland and Dale, 2000).
A wound is defined as a break in the epithelial integrity of the skin. Normal wound healing involves a complex and dynamic but superbly orchestrated series of events including inflammation, new tissue formation and tissue remodeling. Wound healing begins the moment the tissue is injured and requires precise coordination of epithelialization and dermal repair, of which the epithelialization process is ultimately dependent on the migration, proliferation, and differentiation abilities of keratinocytes (Singer and Clark, 1999). During epithelialization, keratinocytes located at the wound perimeter migrate and proliferate to form a single layer over the wound. Further proliferation and differentiation of the keratinocytes establish an epidermal layer comprising the normal stratified layers. Also keratinocytes with normal dermal fibroblasts lead to upregulation of mRNA for collagen type I and III, increased fibroblast proliferation, and extracellular matrix accumulation and remodeling to complete healing by restoring the structure and function of the tissue (Bergers G and Coussens L M, 2000). Thus, the ability of keratinocyte proliferation and migration is essential for performing these processes on the skin surface. With the knowledge that certain growth factors are naturally engaged during wound healing, work has been directed towards developing growth factor-based methods for treating wounds (Mustoe et al., 1994; Steed, 1995). However, most attempts employing such a strategy have failed to achieve clinically significant results, due in part to difficulties associated with use of therapeutic proteins such as the large size of the proteins involved. Use of growth factor therapies also suffers from the complexity and high costs associated with preparing large proteins. Host defense peptides (HDPs), also known as antimicrobial peptides, have been implicated as regulators of cutaneous wound healing. Due to their small sizes these short active peptides have attracted the attention for therapeutics development (Zhang and Falla, 2006).
HDPs are ubiquitous in nature and form central components of the innate immune system of eukaryotes. They are essential to innate host defense as effectors of pathogen clearance as well as modulation of host cell behaviors to promote tissue regeneration and repair. Normal wound repair involves a precise orchestration of inflammation, epithelialization, tissue-granulation and remodeling. Host defense peptides have been shown to influence all of these behaviors. The cathelicidin PR-39 possesses anti-inflammatory function by inhibiting neutrophil oxidase activity, and induces syndecans, heparin sulfate proteoglycans important in wound repair (Gallo et al., 1994; Shi J., Ross, C R. et al., 1996). Another member of the cathelicidin family host defense peptides, LL-37, was also shown to influence the reepithelialization of human skin wounds in organ culture (Heilborn J D, et al., 2003). Human neutrophil defensin promotes the expression of type I collagen while inhibiting the expression of interstitial collagenase (Oono T., et al., 2002). Furthermore, human cathelicidin LL-37 and human β-defensin-3, which are extremely diverse, promote activities including the stimulation of epithelial cell migration, promotion of angiogenesis, and suppression of pro-inflammatory responses (Steinstraesser et al., 2008, 2009). They attract neutrophils, monocytes, mast cells, and T lymphocytes, and also induce the production of neutrophil and monocyte chemoattractants in many cell types. Recently, HDPs have also been implicated as regulators of cutaneous wound repair by modulating inflammation, angiogenesis, and extracellular tissue deposition and remodeling. It has been shown that the influence of HDPs on wound repair is not dependent on antimicrobial function and provides a potential novel clinical application for HDPs. We have previously reported that a non-antimicrobial host defense peptide HB107 (MPKEKVFLKIEKMGRNIRN) (SEQ ID NO.: 10) derived from cecropin B retained the ability to aid in wound repair in a murine model and the benefit observed with HB107 was indistinguishable from the wound treated with growth factor rhPDGF (Lee, et al., 2004). Histological analysis of HB107 treated wounds suggests that epidermal hyperplasia was increased in HB107-treated wounds, an indication that HB107 may influence keratinocyte proliferation or migration (Lee P H., et al., 2004). A new group of synthetic variants of HDPs, termed innate defense regulators (IDRs), which provide broad-spectrum protection against systemic infections with multidrug-resistant bacteria, have recently been described (Easton D M., et al., 2009). For example, IDR-1 and IDR 1002 confer protection against microbial challenges by enhancing innate immune defenses of the host while suppressing potentially harmful excessive inflammatory responses (Easton et al., 2009; Nijnik A., et al., 2010).